Object detection and ranging has traditionally been done by RADAR (Radio Detecting And Ranging) systems. Radar waves have a relatively long wavelength that creates a broad radar beam. In many detecting situations the broad beam covers the entire target, making it difficult to obtain target shape information. Thus, RADAR generally cannot accurately determine the shape of an object in those applications requiring target detection, recognition and identification.
In recent years, LADAR (Laser Detecting And Ranging) systems have been introduced. LADAR systems use laser beams, which have a short wavelength, in place of radio waves. The short wavelength laser beam provides a much narrower beam and higher power with which to illuminate the target. For example, the beam may be less than a meter in diameter. From these many measurements of range, the three dimensional shape of the target may be obtained, thereby allowing the target to be distinguished from non-targets (referred to as “clutter”).
A LADAR system is an electro-optical system using a laser as an illuminator and a receiver which detects a return of the laser and converts the time taken for the return to a range value. Some LADAR systems are of the scanning variety and are referred to as scanning LADAR systems. In this type of LADAR, a laser is pulsed at a high rate with one pulse per pixel (picture element) or portion of the target area to be studied. The laser is scanned such that each pixel in the field of view (FOV) is illuminated and detected. Each laser pulse may be reflected from the target area and received by a detector at the LADAR site. The receiver has a collection aperture and a detector with an instantaneous field of view (IFOV) which corresponds to the portion of the target area which is illuminated by the laser beam. The receiver IFOV is less than the FOV and hence defines the pixel location within the FOV.
The detector and the laser are scanned congruently so that the detector is always positioned to receive any reflected laser beams from the most recent laser pulse. After the entire target area has been scanned, the LADAR system has enough information to determine the approximate range to any object within the target area and the approximate shape of any detected object. Since the LADAR system can be mobile and objects within the target area are often mobile, distortion is introduced into the scanning LADAR system model due to the relative movement of the laser source and the target object during the scan of the LADAR. These distortions are manifested in inaccurate range determinations and imprecise shapes. In addition, purely analog range processors, such as those used with scanning LADAR systems, are not as accurate as all digital or hybrid analog-digital range processors.
Flash LADAR systems have been introduced which solve the problems associated with scanning LADAR systems. Flash LADAR systems illuminate the entire target area with a single pulse from the laser. This laser pulse is then reflected from any object within the target area. Next, the reflected laser pulse is received and the object detected. A flash LADAR system typically uses many detectors arranged in a two dimensional detector array for recreating the target area and for effectively determining the range and approximate shape of any objects contained within the FOV. The range and shape information may be used to identify an object and to determine the location of the object.
General reference in this regard can be made to commonly assigned U.S. Pat. No. 6,392,747 B1, issued May 21, 2002, “Method and Device for Identifying an Object and Determining its Location”, to John B. Allen and Kent McCormack, the disclosure of which is incorporated by reference herein in its entirety.
It is known when digitizing fast laser return pulses, e.g., those with less than one nanosecond (ns) resolution, to use a greater than one gigasample (>109) per second (GSPS) analog-to-digital converter (ADC) per detector array pixel. At present there are commercially available, high speed bipolar-based, 8–10 bit ADCs, with an on-chip sample/hold (S/H) function, that are capable of digitizing at these sample rates.
However, each pixel requires an individual ADC. Since the ADCs are individually packaged, an n-pixel array would require n separate ADC circuit packages or devices. Additionally, these ADCs typically dissipate 0.5 to 6 watts per device, or more, depending on the process technology used. As such, from a focal plane array (FPA) perspective, this approach is only feasible for small linear arrays of detectors.
Furthermore, the ADC data must be demultiplexed (DMUXed) down to a clock rate that is suitable for inputting into slower CMOS signal processors. For example, and considering an 8:1 DMUX function, the ADC data may be slowed to a 125 MHz clock rate, which is acceptable for a conventional CMOS processor. The DMUX function itself, however, represents at least a 3–6 watts power dissipation per ADC.
Based on the foregoing it may be appreciated that this approach requires a large amount of power, and thus inherently causes thermal problems, packaging problems (including packaging parasitic problems), as well cost and complexity problems. As such, severe operating and design constraints are placed on the required LADAR readout integrated circuits (ROICs).
For example, using this approach for a small (e.g., 10×10 detector array), the ADC and DMUX function for generating digital data suitable for processing by a conventional CMOS signal processor is on the order of 600–1200 watts. This figure does not take into account the power dissipated in required I/O circuitry between the DMUXed data and the CMOS signal processor, which can be about 150 watts. The total power, not including the CMOS processor, is then greater than about 750 watts, resulting in the occurrence of the foregoing problems.
By the use of evolving silicon germanium BICMOS process technologies the total required power might be reduced by maintaining the I/O on-chip, and scaling down the ADC and DMUX power. However, the maximum power saving may only approach about 50%, resulting in a required operating power that is excessive for any but relatively small linear detector arrays.
Also, due to the integration levels that are currently achievable, not all of the pixel array electronics can be integrated onto one chip, even for a relatively small 10×10 array. As a result, a multi-chip design is required, that in turn requires careful packaging to avoid parasitic problems resulting from the high operating frequencies.
It can thus be appreciated that a need exists to provide a practical solution to the requirement for increased pulse sampling and resolution in a LADAR system, without incurring the power, size, cost, and performance penalties associated with the conventional approaches.